At least 16 senior members of the al-Qaeda-linked al-Nusra Front militant group have been killed in an airstrike in northwestern Syria, a monitor says.

Rami Abdulrahman, the director of the so-called Syrian Observatory for Human Rights, said on Friday that the air raid targeted a Nusra Front meeting in Abu al-Duhour air base in Idlib Province.

He said the base had been heavily pounded in the attack, which came a day after a number of Syrian civilians were killed by al-Nusra snipers in Kefraya town of Idlib on Thursday.

The neighboring Kefraya and al-Fouaa towns, located in northern Idlib, are besieged by Takfiri groups affiliated to the so-called Jaish al-Fatah terrorist organization.

Fouaa was also targeted on Tuesday with more than 40 militant rocket shells that left five civilians injured.

Syria has been gripped by foreign-backed militancy since March 2011.

The United Nations Special Envoy for Syria Staffan de Mistura estimates that over 400,000 people have been killed in the conflict, which has also displaced over half of the Arab country’s pre-war population of about 23 million.